一、关于block在内存的位置
Block在内存中的位置分三种：
1、NSGlobalBlock:类似函数，位于代码段（未引用外部变量）：
float (^sum)(float, float) = ^(float a, float b){
return a + b;
};
2、NSStackBlock:位于栈内存，函数返回后block将无效（引用外部变量，但只能在该函数作用域内使用，出作用域该变量内存已被释放，出作用域触发block访问该变量会Crash）：
{
NSArray *testArr = @[@"1", @"2"];
void (^TestBlock)(void) = ^{
NSLog(@"testArr :%@", testArr);
};
NSLog(@"block is %@", ^{
NSLog(@"test Arr :%@", testArr);
});
}
3、NSMallocBlock:位于堆内存（对NSStackBlock 使用copy修饰（strong 应该也可以））该类型拥有保存外部变量内存的能力。所以调用外部变量不会crash,下面将讲解对外部变量的存取管理。
二、关于block外部变量的存取管理
#######在这里只讨论ARC机制下
- (void)test
{
int a = 0;
__block int b = 0;
self.globstring = @"1";
NSString *__localString = @"1";
__block NSString *_blockString = @"1";
printf("a address: %p\n", &a); //a address: 0x7fff56516b3c
printf("b address: %p\n", &b); //b address: 0x7fff56516b30
printf("local address: %p\n", &__localString); //local address: 0x7fff56516b00
printf("_block address: %p\n", &_blockString); //_block address: 0x7fff56516af8
printf("glob address: %p\n", &_globstring); //glob address: 0x7fed5bc09520
void (^TestBlock)(void) = ^{
printf("a value: %d\n", a); //a value: 0
printf("b value: %d\n", b); //b value: 1
printf("b address: %p\n", &b); //b address: 0x60800002a7f8
printf("local address: %p\n", &__localString); // local address: 0x6080000979a0
NSLog(@"lockstring is : %@\n", __localString); //lockstring is : 1
printf("_blcok address: %p\n", &_blockString); //_blcok address: 0x6080000470d8
NSLog(@"_blcokstring is : %@\n", _blockString); //_blcokstring is : 2
printf("globaddress: %p\n", &_globstring); //globaddress: 0x7fed5bc09520
NSLog(@"globstring is : %@\n", _globstring); //globstring is : 2
};
__weak typeof(self) this = self;
self.copyBlock = ^{
printf("a value: %d\n", a); //a value: 0
printf("b value: %d\n", b); // b value: 1
printf("b address: %p\n", &b); // b address: 0x60800002a7f8
printf("copy block local address: %p\n", &__localString); //copy block local address: 0x608000099660
NSLog(@"copy block string is : %@\n", __localString); //copy block string is : 1
printf("_blcok address: %p\n", &_blockString); //_blcok address: 0x6080000470d8
NSLog(@"_blcokstring is : %@\n", _blockString); //_blcokstring is : 2
printf("glob address: %p\n", &_globstring); //glob address: 0x7fed5bc09520
NSLog(@"string is : %@\n", this.globstring); //string is : 2
};
a = 1;
b = 1;
printf("a address: %p\n", &a); //local address: 0x7fff56516b3c
printf("b address: %p\n", &b); //_block address: 0x60800002a7f8
printf("local address: %p\n", &__localString); //local address: 0x7fff56516b00
printf("_block address: %p\n", &_blockString); //_block address: 0x60800002a7f8
__localString = nil;
_blockString = @"2";
self.globstring = @"2";
TestBlock();
self.copyBlock();
}
通过观察上面的输出，得出结论 :
1、普通的局部变量，BLOCK声明时会COPY它的值，并且拥有新的地址，也就是说block内部a 地址和外部a地址不同（如上）。互不影响。
2、__block局部变量，BLOCK声明时会在堆中新建一个内存地址，并且BLOCK之后的所有b 或者 _blockString （如上）都是这个地址，所以后续的读写都是再这个地址上进行。会不想影响。即使不触发block 地址也已经不是原来的地址。
3、全局变量 globstring (如上)BLOCK 访问都是 它本身的地址。直接读写操作。
4、BLOCK引用外部对象时会强引用外部变量 ，如果该外部变量也强引用BLOCK就会造成return circle （比如self property block ,block 内部访问self 或者self的属性），为了解决这个使用weakSelf。
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